Type II mucopolysaccharidosis (MPS), also known as Hunter's syndrome, is an inherited metabolic disease caused by a defect in the enzyme iduronate 2-sulfatase (IDS), which functions to degrade mucopolysaccharides. An insufficient level of IDS causes a pathological buildup of heparan sulfate and dermatan sulfate in, e.g., the heart, liver, and central nervous system (CNS). Symptoms including neurodegeneration and mental retardation appear during childhood; and early death can occur due to organ damage in the brain. Typically, treatment includes intravenous enzyme replacement therapy with recombinant IDS. However, systemically administered recombinant IDS does not cross the blood brain barrier (BBB), and therefore has little impact on the effects of the disease in the CNS.